The present invention relates to the altitude compensating apparatus for use with an internal combustion engine, which compensates the decrease of the air to fuel ratio of the mixture, sucked in the engine, caused by the lowering of the density of the air so as to keep the air to fuel ratio at an optimum value when the engine is operated on the high lands.
In a conventional carburetor for supplying the air-fuel mixture to the engine, the fuel is sucked in an air passage in response to the negative pressure produced by the air flow in the air passage, being mixed with the air flowing through the air passage. It is known that the air to fuel ratio of the mixture thus produced decreases or becomes rich as the density of the air lowers. The density of the air is also known to change with the altitude. Therefore, on the high lands the mixture produced in a conventional carburetor and sucked in the engine becomes so rich as to increase the unburned harmful components in the exhaust gases from the engine.